Some people who have read my book Storyteller may have been wondering why there is no mention of Merlin in the early Arthurian tales that Gwernin tells and hears. The answer is simple: in those days he was not part of them.
Merlin, as Arthur’s magician and prophet, was a literary creation of Geoffrey of Monmouth, writing in the 11th century. In his History of the Kings of Britain, Geoffrey took a number of old Welsh legends and reshaped them to his purpose. One of his sources concerned a 6th century British poet / prophet / madman called Myrddin Wyllt, to whom various prophetic poems were attributed. Myrddin (pronounced roughly Merth-in) was supposed to have run mad after a battle in which his lord was killed, and to have lived thereafter as a wild man in the Caledonian Forest, subsisting on apples and acorns, and emitting poetry and prophecies. Using his own writer’s magic, Geoffrey changed the madman into a magician, and re-christened him Merlin, possibly because of an unfortunate resemblance of his original name to a French word for excrement.
Taliesin, who instead of Merlin is associated with Arthur in early Welsh sources, also gained a reputation as a prophet, and the 9th century poem Ymddiddan Myrddin a Thaliesin features a dialogue between him and Myrddin, in which they take turns uttering prophecies in their characteristic verse styles. In my tales I've given the historical Taliesin parts of what was later Merlin’s role.
After all, he was there first.